Not Applicable
Not Applicable
1. Field of the Invention
The present invention relates to hydraulic circuits that operate machinery; and more particularly to controlling the pressure and flow of hydraulic fluid supplied to power actuators of that machinery.
2. Description of the Related Art
A wide variety of machines have working members that are driven by hydraulic cylinder and piston assemblies. Each cylinder is divided into two internal chambers by the piston and selective application of hydraulic fluid under pressure to either of the chambers moves the piston in a corresponding direction. While that action is occurring, fluid is being drained or exhausted, from the other,cylinder chamber to a tank for the hydraulic system.
Traditionally the flow of hydraulic fluid to and from the cylinder was controlled by a manually operated valve, such as the one described in U.S. Pat. No. 5,579,642. There is a trend away from manually operated hydraulic valves toward electrically controlled solenoid valves. This change in technology facilitates computerized regulation of various machine functions. Electrical control also simplifies the plumbing of the hydraulic system, as the control valves can be located near each cylinder and not at the operator station. Thus only a single pair of pump and tank lines needs to be run to the hydraulic actuators throughout the machine. Although separate electrical wires may have to be run to each valve, those wires are easier to run and maintain as compared to hydraulic lines.
Electrically controlled metering valves have a potential problem of not closing when commanded because an obstruction across a metering element due to fluid contamination causes the solenoid armature to hang up. Under that circumstance, control of the cylinder and of the machine member operated by the cylinder are lost. This can create a potentially hazardous situation where an open valve allows fluid to drain from the cylinder causing the machine member to drop by gravity.
Another condition occurs where a single pump provides pressurized fluid to several functions on the machine. For example, an excavator has a boom coupled to an arm that has a movable bucket at a remote end. Each of these three components is operated independently by a separate hydraulic cylinder. During complex motion, the boom may be lowering by gravity with the exhausting hydraulic fluid draining directly to tank, while the arm is being powered by pressurized fluid from the pump. In this situation, energy in the exhausting fluid is being lost and additional power has to be consumed by the pump to provide the pressurized fluid for operating the arm and possibly other functions on the machine. This limits the rate of those powered functions and corresponding slows work function cycle time. Thus there is a degree of inefficiency to this operation.
A further concern in hydraulic systems is that some valves are sensitive to the pressure drop across their metering elements. Specifically, the resolution of the metering may be compromised as the pressure drop increases. FIG. 1 illustrates the typical relationship between the electrical current applied to the valve actuator and the flow rate of fluid through the valve at different pressure drops across the valve. As can be seen, a change in the actuator current from level I1 to a higher level I2 produces a relatively small change in the flow rate when the pressure differential is relatively low, for example 20 bar. In contrast at a greater pressure drop, such as 200 bar, the same change in valve actuator current (I1 to I2) produces a much greater change in the flow rate. In other words, the lower the pressure drop across the valve element, the resolution of flow metering becomes finer.
As a consequence, a small error in the control of the actuator current or a small change in the valve response can have a dramatic impact on the flow rate at higher pressure drops. This can result in a significant difference in the movement of the machine member being controlled by the valve. Thus, if fine metering control is desired, the pressure drop across the valve has to be maintained at a relatively small level, or very accurate control of the actuator current must be accomplished.
The present invention provides an improved hydraulic system that addresses each of these concerns.
That hydraulic system has a source of hydraulic fluid under pressure and a tank for storing hydraulic fluid. A shared fluid return line is connected to the tank by an electrically driven return line valve. A source sensor provides a signal that indicates the pressure of the hydraulic fluid from the source and a tank sensor produces another signal denoting the pressure in the shared fluid return line.
A plurality of hydraulic functions are connected to the source of pressurized fluid and the shared fluid return line in order to operate mechanical members on a machine. At least one of those hydraulic functions comprises an actuator, such as a bidirectional hydraulic cylinder, with first and second ports. A first control valve connects the source to the first port of the actuator and a second control valve couples the first port to the shared fluid return line. A third control valve governs fluid flow between the source and the second port of the actuator, while a fourth control valve connects the second port to the shared fluid return line. This function also has a first sensor which generates a signal indicating the hydraulic pressure at the first port, and the pressure at the second port is evidenced by a signal from a second sensor.
An electronic controller has inputs connected to the source sensor, tank sensor, first sensor and the second sensor and has outputs connected to the first, second, third and fourth control valve, as well as the return line valve. The controller operates selective ones of the control valves to produce desired amounts of movement of the actuator. The controller responds to the pressure indicating signals from respective ones of the sensors by operating the return line valve to control the pressure in the shared fluid return line.
The hydraulic system has several regeneration modes of operation in which fluid being exhausted from one port of the actuator is supplied into the other actuator port. This regeneration either eliminates or drastically reduces the amount of hydraulic fluid that must be supplied from the source to the actuator. Thus the amount of energy needed to power the source of pressurized fluid and the time to accomplish function operations are reduced. In the regeneration mode of gravity lowering (potential energy) or inertia braking (kinetic energy), make-up fluid is obtained from another hydraulic function on the machine via the shared fluid return line to feed into a port of the actuator. In these regeneration modes the controller operates the return line valve to restrict fluid from flowing into the tank from the shared return line, so that fluid will be available to be supplied into an actuator port.
The return line valve also is operated to pressurize the shared fluid return and decrease the pressure drop across a control valve. By reducing the pressure drop, the flow metering resolution of that control valve is improved for better control of the actuator. Metering improvement also can be regulated within the four way valve.